The invention relates to a reflecting image projection screen comprising a polarizer and a reflecting layer for reflecting radiation which is incident on the reflecting layer via the polarizer.
The invention also relates to an image projection system comprising such a reflecting image projection screen.
An image projection system having a reflecting image projection screen of the type described in the opening paragraph is known from European Patent Application EP 0,295,913 A2. The image projection screen described in this Application is a reflecting image projection screen and may be implemented, for example as a metal screen. Since not only radiation supplied by the image projector, hereinafter referred to as signal light, but also ambient light is incident on the image projection screen and is reflected by this screen into the audience room, the ambience of this screen is to be considerably darkened in order that an image which is rich in contrast can be observed. For this reason the side of the image projection screen facing the viewer is provided with a polarizer which absorbs substantially 50% of the ambient light. In this manner the reduction of contrast of the image due to ambient light is reduced. The illumination system of the image projector comprises a radiation source whose radiation is split into p and s-polarized radiation by a polarization-sensitive beam splitter. One of the two linearly polarized beams is incident on a liquid crystal element in which the direction of polarization of the beam is modulated in conformity with the image generated by the element. Subsequently, the modulated radiation is projected in the form of an image on the image projection screen via a projection lens. Since the projected signal light is linearly polarized in the direction of polarization of the screen polarizer, this light is substantially completely passed to the audience room by the polarizer on the screen after reflection on the reflecting layer.
In the system described radiation incident on the image projection screen is reflected at an angle whose size is substantially equal, but opposite to the angle of incidence. This is the case, inter alia, in image projection systems in which the image projector is secured, for example to the ceiling and in which the screen is a specularly reflecting screen, for example a metal screen. The radiation is then reflected towards the audience room which is at a lower level than the projection screen.
Due to the required mutual positioning of the projector, the projection screen and the viewer(s), a metal screen is applicable to a limited extent only. In practice it is generally desired to have more freedom in the positions of said system components and the viewers. A frequent arrangement is the one in which, for example, a portable projector is placed at the viewers' eye level, while it is further desired that the projected image has a good quality from different positions, notably in the horizontal plane.